The 80th anniversary of one of the most controversial maritime disasters in recent British history has been marked by a Southampton-based maritime charity.

Sailors’ Society lowered its flag to half mast to commemorate the 99 lives lost in the HMS Thetis tragedy. The submarine sank during diving trials off Liverpool Bay on June 1, 1939.

As well as the normal complement of 59 men, the vessel was carrying observers from the Admiralty, the ship builders Cammel Laird and other personnel.

In total, 103 people were on board.

On the vessel’s first dive, it sank to the seabed.

Only four men managed to use the escape chamber and, even though the vessel’s stern surfaced during the night, the decision was taken by the Admiralty not to drill air holes in the hull so not to damage the vessel’s integrity.

The 99 men who remained on board were overcome by carbon dioxide poisoning – killed slowly by the air they exhaled.

A statement was issued two days later, “The Admiralty regrets that hope of saving lives in the Thetis must be abandoned.”

Sailors’ Society, a charity which supports seafarers and their families, sent wreaths to each funeral, with some laid on the coffins of those who had not been identified.

Mrs West, one of the charity’s port missionaries at the time, counselled the bereaved families as they arrived in Holyhead.

Sandra Welch, Sailors’ Society’s acting CEO, said, “In lowering our flag, we remember those lost in the tragedy, the families they left behind and those who cared for them in their time of need.”

Although a major incident at the time, the tragedy was immediately overshadowed by the onset of World War Two.

Having been salvaged, the Thetis was re-commissioned as HMS Thunderbolt, only to be sunk off Sicily in March 1943 with the loss of all hands.

On 7 November 1947, a small memorial was dedicated in Maeshyfryd Cemetery to the 99 lost souls of the Thetis and the hundredth man, navy diver Petty Officer Henry Perdue, who died of the bends during the salvage operations.